1. Field of the Invention
The present invention relates generally to electric motors and, more particularly, to control systems for electric motors.
2. Description of the Related Art
In many modern control systems, performance parameters (e.g., speed and torque) of electric motors (e.g., AC induction motors, brushless DC motors and synchronous motors) are controlled in accordance with space vector modulation which generates a reference voltage vector Vref whose angular speed determines a desired synchronous speed of a motor and whose amplitude determines a desired motor voltage.
The reference voltage vector Vref rotates with respect to a plurality of voltage vectors that each represent a respective one of switch states of an inverter that is coupled to the motor. The control system commands the inverter to generate patterns of the switch states as a function of the instantaneous magnitude and location of the reference voltage vector Vref. Feedback signals are often provided to the control system by a single current sensor that is coupled to one side of the inverter.
Although space vector modulation facilitates improved control performance (e.g., less harmonic distortion) and the single current sensor reduces system costs, it has been found that these systems sometimes contain errors which degrade system performance.
The present invention is directed to space vector modulation methods and structures that reduce feedback errors which degrade motor control performance.
The invention recognizes that these errors generally occur when feedback signals become so narrow that they cannot be accurately processed. The invention further recognizes that narrow feedback signals correspond to certain relationships between a reference voltage vector Vref and voltage vectors that each represent a respective one of converter switch states.
In response to these recognitions, embodiments of the invention provide space vector modulation methods and structures which modify the reference voltage vector Vref to thereby avoid excessively narrow feedback signals. In particular, the rotation of a reference voltage vector Vref is modified as it rotates with respect to voltage vectors that each represent a respective one of converter switch states in an electric-motor control system. One embodiment modifies the reference voltage vector Vref as it approaches various voltage vectors and another modifies the reference voltage vector Vref when its magnitude is less than a predetermined magnitude.